In the most widely used (main-stream) form of color photography a blue recording yellow image dye-forming layer unit, a green recording magenta image dye-forming layer unit, and a red recording cyan image dye-forming layer unit are coated as superimposed layers on a photographic film support to form a color photographic film. With the color film mounted in a camera, light from a photographic subject is directed through a lens to the topmost of the superimposed layers and penetrates each of the layer units in the same area of the film. Differentially sensitized silver halide grains in the layer units cause three superimposed latent images to be formed, each representative of exposing light from a different one of the blue, green and red regions of the spectrum.
To obtain a viewable color image the film is processed in a sequence of processing baths, starting with a color developer. The latent image bearing silver halide grains are selectively reduced to silver by a color developing agent, which, in its resulting oxidized form, reacts with a dye-forming coupler to produce image dye. After development, the developed silver is reconverted to silver halide in a bleach bath, and the silver halide is then removed by in a fixing bath to render the color film light insensitive. Superimposed yellow, magenta and cyan dye images are left in the film at the conclusion of processing, corresponding to the image patterns of blue, green and red light exposure, respectively.
In most instances negative-working silver halide emulsions are employed, and the dye images are negative images. To obtain a viewable positive color image a color paper (having the same types of layer units described above, but coated on a white reflective support) is exposed by white light passing through the image bearing color film. Instead of exposing the color paper through the processed color film image, it is possible to retrieve the dye image information from the fully processed color film by scanning. This information can be stored in a digital computer and used in various ways--e.g., for viewing on a cathode ray tube (CRT) monitor or for controlling laser or photodiode exposure of a color paper. To produce a viewable image in the color paper, it is processed in a series of aqueous baths as described above.
It should be noted that the color film as typically used in a camera to create an original image of a photographic subject is an "image capture" film. Here subject motion and/or limited light availability can place high demands on imaging speed. On the other hand, the color paper is an "output" medium that produces an image from an image already captured. As an output medium color paper is exposed without subject motion and with controlled lighting. The standard practice is therefore to select output media of much lower imaging speeds than desired in most image capture films.
The high levels of internal amplification afforded by converting a latent image site on a silver halide grain formed by a few captured photons into thousands of dye molecules allows extremely high levels of imaging sensitivity to be attained in the color film. This, more than any other single factor, accounts for the widespread use of silver halide color film for image capture.
There are, however, many limitations and disadvantages of silver halide color films. One of the disadvantages that has been most vigorously addressed is the need to employ aqueous baths for processing. Color image transfer systems have been developed for integrating processing compositions into the film package. However, the reduction of image sharpness during dye image transfer has precluded the use of these systems in the overwhelming majority of photographic applications in which the color image is significantly enlarged for viewing. Attempts to produce acceptable viewable image sizes without enlargement have resulted in cameras for color image transfer systems being bulky and unattractive to users.
Photothermographic elements rely on light for latent image formation and uniform heating to produce a viewable image. While photothermographic elements eliminate aqueous processing baths, pronounced limitations have restricted their widespread use to black-and-white (silver) imaging. A summary of photothermographic element constructions is provided by Research Disclosure, Vol. 170, June 1978, Item 17029. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England.
Photothermographic imaging systems are more complex than corresponding aqueous processed imaging systems. In a typical form an imaging layer unit contains (a) photosensitive silver halide grains formed in situ or ex situ, (b) an oxidation-reduction image forming combination comprising (i) a metallic salt or complex of an organic compound as an oxidizing agent and (ii) an organic reducing agent or developing agent, and (c) coating vehicle. Although latent image formation still relies on silver halide grains, the actual mechanism of image formation is quite different than in main-stream silver halide photography. In fact, relatively low imaging speeds have generally limited photothermographic elements to output imaging applications.
It has been recognized that photothermographic elements can be constructed to produce dye images, as illustrated by Research Disclosure, Item 17029, cited above, XV Color Materials. When a photothermographic film is constructed with three superimposed image dye-forming layer units, each of the layer units contains developed silver, formed either as a result of imaging or by the spontaneous reduction of silver halide to silver (i.e., fog and printout). To avoid high levels of minimum density superimposed on the image dye densities, the art has moved in the direction of transferring the dye image to a separate receiver. This eliminates only one of the limitations of photothermographic imaging while embracing all of the limitations of image transfer systems. Color photothermographic image transfer systems are illustrated by Clark et al U.S. Pat. No. 4,504,568 and Bailey et al U.S. Pat. No. 5,468,587.
All of the color imaging elements described above capable of replicating natural colors coat three superimposed image dye-forming layer units on a support. This arrangement, as well as employing three different image dye-forming materials to produce yellow, magenta and cyan dye images, has been considered essential to achieving acceptable natural color images. It is easily recognized that the coating of image dye-forming layers in a superimposed relationship degrades the sharpness of the dye image in the underlying layer units. Also, not only is a minimum of three layers required to be coated, but in most preferred constructions interlayers further increase the number of layers that must be coated.
Tabular grain emulsions are well known for use in main-stream photography, as illustrated by Kofron et al U.S. Pat. No. 4,439,520. Reeves U.S. Pat. No. 4,435,499 demonstrated increased development efficiency for tabular grain emulsions in photothermographic elements. Although occasionally mentioned as a possible alternative grain selection for photothermographic elements, tabular grain emulsions have not been identified as the silver halide emulsions of choice for photothermographic elements.